Obesity, often the result of a high fat diet, leads to chronic inflammation and stromal cell proliferation. In this application, we will address the issue of how a high fat diet leads to enhanced tumor cell growth and approach this from a previously unexplored direction. Examination of an adult mammalian regeneration model employing the MRL/MpJ mouse studied for its susceptibility to autoimmunity, obesity, and enhanced wound healing abilities, displays multiple phenotypes similar to the tumor microenvironment and has been the subject of study in the Heber-Katz laboratory for over a decade. The mapping of genes in this mouse has led to the identification of a novel gene signature defined by a small subset of genes which are regulated by a high fat diet in a sexually dimorphic fashion. The genes that constitute this so-called high fat-selected regeneration-associated or HFSRA gene signature are the basis of the proposed approach to defining the mechanistic basis for the association between a high fat diet and breast cancer. This approach is strongly supported by results demonstrating that these same genes are similarly expressed in human breast cancer-derived stromal cells. These studies will utilize the 4T1 orthotopic mouse breast cancer model in syngeneic BALB/c mice in vivo. In vitro studies will be carried out using 3D-culture systems relevant to the tumor microenvironment based on an understanding of the complexities of the tumor microenvironment and the host response to tumors, ranging from the role of stromal cells, inflammation and vascularization to tumor immunity, and is the focus of the Pur? laboratory. Working together as a team, the combined expertise of Drs. Heber-Katz and Pur? render the important but complex goals of this project eminently feasible.